In many industrial operations such as papermaking or mining, or in many wastewater treatment applications such as sludge dewatering, the flocculation of an aqueous suspension is necessary or desirable. Polymers derived from acrylamide and cationic or anionic monomers such as N,N-dimethylaminoethyl acrylate methyl chloride quaternary salt or acrylic acid, respectively, are widely used to improve the flocculation performance parameters in these operations. Examples of performance parameters often measured are first pass retention (papermaking), settling rates (mining), and turbidity reduction (sludge dewatering). Terpolymers of acrylamide containing both cationic and anionic charge within the same polymer are termed poly(ampholytes) and have also been applied extensively in these systems. Such poly(ampholytes) are prepared by polymerizing acrylamide with a monomer containing a cationic group and another monomer containing an anionic group. Amphoteric water-in-oil self-inverting polymer emulsions and their use in papermaking are disclosed in U.S. Pat. Nos. 4,330,450, 4,363,886 and 4,392,917.
All of the flocculant chemistries described above possess a net ionic charge within a polymer chain either universally (as with copolymers when a comonomer has a net charge) or locally (as with polyampholytes). In the case of amphoteric terpolymers, the differences in monomer incorporation rates along with statistical variability creates non-homogeneous local charge distributions, giving rise to a net cationic or net anionic charge for a given polymer chain. The net charge within a given polymer segment is important, because any local positive or negative charge will be attracted to surfaces of opposite charge. Polymer segments containing a net charge will collapse or become less soluble at higher ionic strengths due to a reduction in electrostatic repulsion. Such a reduction in polymer solubility dramatically reduces the effectiveness of flocculation. For example, in papermaking systems, cationic flocculants are often used to bind to anionic filler and fiber surfaces. Similarly, cationic flocculants can bind effectively to anionic clay surfaces in mining applications.
In many process waters, there may be anionic substances that compete with the target anionic materials to be flocculated. When the target filler and fiber surfaces must compete for polymer with anionic solutes or colloids (anionic "trash"), the flocculation efficiency is dramatically decreased because of the reduction in available cationic sites within the polymer. To circumvent this problem, one has traditionally "titrated" the detrimental anionic substances using a cationic coagulant. When the coagulant neutralizes the anionic charge on the high surface area of the detrimental anionic substances, the flocculant then remains "free" to aggregate the remaining anionic fiber and filler surfaces, which is the desired result. However, fluctuations in charge demand of the process water owing to varying levels of the soluble detrimental anionic substances means that the coagulant dose will have to change in order to achieve the same extent of neutralization. Although eliminating the variations in flocculation performance is critical to maintaining process control and consistent operation, maintaining a constant solution charge prior to flocculent addition can be quite difficult in practice. An alternative approach to charge neutralization using coagulant addition is adding a flocculant which, by design, is resistant to changes in anionic trash levels and concentrations of charged species in solution.
One example of a flocculant more resistant to anionic trash is poly(acrylamide), which is a neutral, uncharged homopolymer. One can also manufacture a polymer which, rigorously, is overall electrically neutral but which contains both cationic and anionic functionality by incorporating a zwitterionic monomer.
Zwitterionic monomers and polymers have been widely studied, and their unique properties in aqueous solution have been well documented. For example, zwitterionic sulfobetaine monomers such as 1-(3-sulfopropyl)-2-vinylpyridinium betaine are commercially available. Vinylpyridinium carboxybetaine monomers are disclosed in J. Poly. Sci., 1957, 26, 251. Zwitterionic monomers based on phosphorous such as 2-methyacryloyloxyethyl phosphorylcholine and 2-[3-acrylamidopropyl)dimethyl ammonio]ethyl 2'-isopropyl phosphate are disclosed in Polymer Journal, 1990, 22(5), 355-360 and Polymer Science: Part A: Polymer Chemistry, 1996, 34, 449-460, respecitively. Vinylimidazolium sulfobetaines and their polymers are disclosed in Polymer, 1977, 18, 1058, and Polymer, 1978, 19, 1157. Carboxybetaines based on sulfonium acrylate monomers are disclosed in U.S. Pat. Nos. 3,269,991 and 3,278,501. Diallyl sulfobetaine monomers and polymers are disclosed in U.S. Pat. Nos. 4,822,847 and 5,788,866. A copolymer of acrylamide and 3-(2-acrylamido-2-methylpropanedimethylamino)-1-propanesulfonate is disclosed in McCormick, et al., Polymer, 1992, 33, 4617. However, in McCormick the polymers are prepared as homogeneous solutions in water, and polymerization is stopped at low conversion of monomer owing to unmanageable viscosities. Flocculation/coagulation studies using zwitterionic homopolymers of much lower molecular weights have been reported in Polymer Prep., 1991, 32, 323 and Environmental Technology, 1998, 19, 323.